This invention relates to an electrolytic polishing apparatus having a magnetic stirrer. Such an apparatus is known from the French patent specification No. 1,131,092 and comprises a container, in which the magnetic stirrer is arranged and which has a cover with a polishing opening, against which a polishing sample can be pressed. In such a polishing apparatus the magnetic stirrer serves to set the electrolyte in rapidly rotating movement, whereby it is possible to obtain a so strong flow of the electrolyte along the surface of the polishing sample that the conditions necessary for the polishing effect (so-called polishing film) are not disturbed, while the gas bubbles liberated by the electrolysis are nevertheless removed. Thereby metal is dissolved from the surface of the sample in such a manner that a uniform and shiny surface is obtained, in which the structure of the metal can be observed without being distorted by the deformations inavoidable in the case of mechanical polishing.
The French patent specification shows two embodiments. In one embodiment the cover is placed sealingly on top of the container and, besides the polishing opening, is constructed with a middle opening. Since electrolyte may leak out between the polishing sample and the cover, the latter is so shaped that the leaking electrolyte can trickle down towards the middle opening. To obtain this, the cover is provided at least in the area of the polishing opening with a protruding edge, and this prevents use of the apparatus for polishing a sample extending beyond the edge of the cover. In the other embodiment, the cover is constructed in the form of a bell which rests on the bottom of the container and the outer diameter of which is somewhat smaller than the inner diameter of the container so that an annular space is formed which can receive leaking electrolyte. Owing to the stirring a pressure above that of the atmosphere is created in this annular space and the edge of the container must therefore extend upwardly beyond the upper face of the bell-shaped cover so that it is not possible in this case either to polish samples which have a size so as to extend beyond the edge of the container. A drawback of both embodiments is that it is difficult to replace the electrolyte by another one when the polishing of a great number of samples is to take place in a running operation. Moreover, it is to be mentioned that there is little possibility of dissipating the heat produced by the electrolytic polishing. To-day it is a desideratum to be able to polish surfaces of 10-20 cm.sup.2, which requires currents of up to 20-25 Amp., whereby considerable quantities of heat are developed during the process. If there are no good possibilities of dissipating this heat, the capacity of the polishing apparatus is reduced.